The invention relates to treads for the manufacture of new tires or for the recapping of tires, and in particular to the patterns for said treads which comprise a large number of cutouts in the form of grooves and/or incisions. It also relates to a production method making it possible to produce such treads.
In order to adapt the performance of tires to the increasingly improved performance of new vehicles, it is becoming necessary, inter alia, to increase the level of the adhesion performance of the tires on a wet road, without for all that adversely affecting the other types of performance thereof. xe2x80x9cAdhesionxe2x80x9d is understood to mean both the adhesion properties of the tire in the direction transverse to the displacement of the vehicle (cornering ability) and those of the tire in the direction longitudinal to the displacement of the vehicle (possibility of transmitting a braking or driving force to the ground).
In order to increase the adhesion potential of a tread of a tire traveling on a road covered with water, it is known to provide this tread with a pattern formed of a plurality of cutouts formed at greater or lesser depths in said tread, said cutouts opening on to the surface of said tread in contact with the road (this surface is called the rolling surface).
xe2x80x9cCutoutxe2x80x9d quite obviously means formed in the tread, be it by removing material once the tread has been vulcanized or be it by molding in a mold for molding said strip and comprising molding elements which project on the molding surface of said mold, each molding element having a geometry identical to the geometry of the desired cutout. As a general rule, a cutout made in a tread is defined by at least two walls of rubber which face one another, said walls being separated by an average distance representing the width of the cutout, the intersection of said walls with the rolling surface forming rubber ridges. Several types of cutout can be distinguished, for example:
grooves or furrows characterized by a width greater than about 10% of the thickness of the tread;
incisions of relatively low width compared with the thickness of the tread; under certain conditions of stress, these incisions may close, at least partially, in contact with the road;
the walls facing one another come into contact with one another at least over a more or less large part of the surfaces of said walls (the ridges formed by an incision on the rolling surface are in contact, which causes the incision to close).
Some cutouts may open into at least one other cutout. The trace of a cutout on the rolling surface of a tread follows an average geometric profile determined as the geometric profile located at an average distance from the ridges formed by the walls of said cutout on the rolling surface. The center axis of the trace of a cutout on the rolling surface corresponds to the straight line of the least-error squares of the distances of the points of the average profile from the trace of said cutout. Furthermore, it is usual to define the groove ratio of a pattern as the following ratio: area of the cutouts on the rolling surface divided by the total area of contact between the tire and the road.
By effecting a plurality of cutouts which open on to the rolling surface, a plurality of rubber ridges is created to break up the layer of water which may be present on the road, so as to keep the tire in contact with the ground and to create cavities forming channels intended to collect and remove the water present in the zone of contact of the tire with the road since they are arranged so as to open outside the zone of contact.
An example of such a pattern is found in U.S. Pat. No. 1,452,099, which describes a tread provided with a plurality of regularly spaced incisions of transverse orientation.
However, the increase in the number of cutouts rapidly results in a substantial decrease in the rigidity of the tread, which has an adverse effect on the performance of the tire, and even on the adhesion. xe2x80x9cRigidity of the treadxe2x80x9d is understood to mean the rigidity of the tread under the combined actions of compressive stresses and shearing stresses in the region affected by the contact with the road. Conjointly, the presence of numerous cutouts forming channels for evacuating water results in a level of traveling noise on a dry road which is nowadays considered as a nuisance which it is desired to reduce to as great an extent as possible, very particularly on vehicles of recent design. This traveling noise is amplified by the cyclical movements of closing and opening of the cutouts which are associated with the friction of the walls of said cutouts when they are closed.
In French Patent 1 028 978, a solution to this problem is, proposed which consists in providing the tread with a plurality of circumferential incisions of low depth over the rolling surface of the new tread so as to increase the flexibility of said tread solely in the vicinity of the rolling surface.
However, since the tire, once mounted on a vehicle, is intended to provide good performance during the entire life of said tire (that is to say, until its tread has worn down to a level corresponding at least to the legally permitted level), it is necessary to provide a tread having a pattern which ensures the lasting quality of the adhesion performance on wet ground.
The object of the present invention is to develop a tread for a tire which manages to combine a very good level of adhesion to a wet road and to a dry road with a low emission of noise while traveling when new and during at least a major part of the life of said tread. Complementary, one of the objects of the invention is to control the evolution of the performance with the wear of the tread.
According to the invention, a tread is proposed, of thickness E in a rubber mix, intended to be placed radially on the outside of a tire, said tread being provided with a rolling surface which is intended to come into contact with the road during the travel of the tire. The tread is provided with a large number of cutouts such that a plurality of cutouts is affected by the zone of contact of the tire with the road. Each cutout is defined by the space enclosed mainly between two opposing walls, said walls being perpendicular to or oblique to the rolling surface, and each cutout has a depth at most equal to the thickness E of the tread, the depth of said cutout being measured as the distance in a radial direction between the points of the contour of said cutout which are farthest from the rolling surface of the new tire and said rolling surface. Furthermore, two main walls of at least one cutout located in the part of the tread affected by the loading on the ground of the tire during travel are connected by at least one rubber connecting element.
The reference SE is given to the total connection surface on each of the walls of said cutout equal either to the total intersection surface of the connecting element in the case of a single element or to the total of the intersection surfaces of all the connecting elements in the case of several elements, and the total surface of each of the main walls of said cutout is referenced ST.
The tread according to the invention is characterized in that:
the two main walls of at least one cutout located in the part of the tread affected by the loading on the ground of the tire during travel are connected by at least one rubber connecting element, the connecting element(s) having with each of the walls a connection surface SE corresponding to the total of the intersection surfaces of each connecting element, said surface SE on each of the walls being at least 10% of the surface ST of said wall,
all the points of the outer contour of the surface of intersection of at least one connecting element with one of the walls are located at a distance from the rolling surface which is strictly less than the depth of the cutout,
and in that, on each of the main walls, the connection surface SE is at most 80% of the surface SG, the surface SG being equal to the surface, measured on said wall of the cutout, defined by the geometric contour L of minimum length traced on said wall and enveloping the connection surface SE.
For a wall, the geometric contour L of minimum length enveloping the total connection surface SE on this wall corresponds to the contour which can be traced on said wall so as to envelop all the total connection surface formed by the intersection surfaces of all the connecting elements with said wall. As a cutout is defined as being the continuous space defined by at least two main walls facing one another, the presence of at least one connecting element between said walls, according to the invention, does not interrupt the continuity of this space, whatever the level of wear of the tread comprising such a cutout.
Advantageously, the effect on the adhesion and noise performance is the more significant the larger the number of cutouts forming the pattern of a tread for a tire are provided with at least one rubber connecting element so as to obtain both an outstanding adhesion performance without for all that adversely affecting the level of performance in terms of traveling noise.
Advantageously, and in order to obtain an optimum connection effect between the walls of the cutouts and a sufficient length of rubber ridges, each surface SG defined by the geometric contour L of minimum length and enveloping the total connection surface SE on one main wall of a cutout is at least 15% of the surface of the corresponding wall ST. Preferably, the total connection surface SE over at least one main wall of at least one cutout is at most 80% of the surface of the corresponding wall ST, so as to preserve a volume of cutout imparting to the tread sufficient adhesion characteristics.
The tread according to the invention has several advantages:
for cutouts oriented substantially transversely to the longitudinal direction of the tread, the impacts of the rubber ridges on the road are reduced, which results in substantial reduction of the noise emitted during traveling, and this despite a relatively large number of ridges;
on emerging from the area of contact, the presence of connecting elements between the walls of the incisions oriented transversely and possibly between the opposing faces of the relief elements limits the oscillating movements of the rubber elements of the tire emerging from said area which cause noise-generating vibrations, thus reducing the noise resulting from these vibrations;
the reduction of the noise component also results from the reduction of the friction of the walls of the incisions on one another when passing into contact, the connecting elements preventing both the moving-together of the walls and the movement of one relative to the other;
a very marked improvement in adhesion, whilst maintaining a large surface of rubber to be worn in the case of a plurality of incisions of low width, said incisions being provided with at least one connecting element.
Overall, the sources of noise which are associated with such a pattern are far less strong owing to the reduction of movement of the walls of the cutouts resulting from the presence of the connecting elements between said walls compared with a pattern which would comprise the same cutouts without any connecting element.
It goes without saying that a tread pattern according to the invention may combine both a plurality of cutouts provided with at least one connecting element with a plurality of cutouts without a connecting element, the proportion between said cutouts possibly being, for example, a function of the type of tire comprising said pattern.
The applicant has surprisingly obtained very good results when using tires provided with a tread of thickness E, provided with a plurality of cutouts oriented virtually in the same direction, each of said cutouts being defined by the space defined principally between two opposing walls, said walls being perpendicular to or oblique to the rolling surface, and said cutouts having a depth h.
xe2x80x9cDepth h of a cutoutxe2x80x9d is understood to mean the maximum radial distance measured between the ridge of the cutout closest to the rolling surface of the tread when new and the point(s) of the walls of the cutout which is or are radially farthest from said rolling surface. This depth h represents the maximum radial distance between the radially farthest points of the walls of a cutout, and is at most equal to the thickness E of the tread.
Furthermore, the tread according to one variant of the invention is such that:
the two main walls of said cutouts are connected by at least one rubber connecting element;
the connecting rate TP=SE/ST, SE for each cutout is at least 0.10 and at most 0.80, SE being equal to the total intersection surface of the connecting element(s) on each of the walls, ST representing the total surface of each main wall of said cutout;
the ratio p/h, between the average pitch p between each of said cutouts and their depth h, is at least 0.2 and at most 1.9.
In order to avoid the appearance of irregular wear of the tread and if a value of the connecting rate TP of the cutouts is fixed, it is then preferable to select the pitch p such that the ratio p/h, in the new state, satisfies the following equation:       p    h    ≥            1      5        ⁢                  (                  1                      T            P                          )            0.75      
Complementary, and in order to achieve a good adhesion performance, it is judicious to select the pitch p such that the ratio p/h satisfies the following equation:       p    h    ≤      2    ⁢                  (                  1          -                      T            P                          )            0.5      
It is also possible to make allowance for the average width e of the cutouts (that is to say, the average distance between the main walls of said cutouts) arranged regularly according to a pitch p on a tread so as to obtain the connecting rate Tp which makes it possible to achieve good results during traveling. It has has been found that good adhesion and wear results are obtained when the connecting rate Tp is at least 0.10 and is at most equal to the following value:   1            (              1        +                              1            3                    ⁢          ε                    )        0.75  
in which xcex5=(pxe2x88x92e)/h.
Once the connecting rate TP is less than about 0.10, the object cannot be achieved because the connecting elements do not supply sufficient rigidity, and cannot sufficiently block the movements of the walls of the cutout to which they are connected; preferably the connecting rate TP is greater than 0.25.
On the other hand, when this rate exceeds the limit value proposed, the rigidity becomes too great and the length of active ridge is too small to maintain satisfactory adhesion performance. xe2x80x9cLength of active ridgexe2x80x9d of a tread of a tire is understood to mean the total of the lengths of all the rubber ridges in contact with the road in the imprint and for a given level of wear of the tread.
Furthermore, and in order to maintain a sufficiently constant performance with the wear of the tread, it is judicious to provide for the value of the connecting rate TP=SP/ST, evaluated for different levels of wear of the tread to decrease substantially regularly with the wear of the tire, at least starting from a predetermined partial level of wear SP representing, on one of the walls of said cutout, the connection surface remaining after partial wear of the tread and ST representing the total remaining surface of said wall corresponding to the same level of partial wear of the tread. In this manner, the effect of the connecting elements on the rigidity is at a maximum when the tread is new, but decreases gradually with the increase in rigidity of said tread resulting from wear.
On the other hand, tire rolling tests have shown that the sculpture according to the invention results in:
an improvement in wear by reducing the sliding of the rubber against the road, be it at the moment of emerging from the zone of contact for transverse cutouts or when cornering for circumferentially oriented cutouts;
an improvement in the fatigue strength of the base of the cutouts (that is to say, the part of the cutouts which is radially farthest to the inside of the tread);
lower retention of stones in the cutouts provided with connecting elements.
It is also noted that the draining ability of the tire pattern according to the invention, that is to say its ability to evacuate the water present on the road, is sufficient whatever the level of wear of the tread, which is a particularly attractive advantage for the user.
In order to obtain long-lasting efficiency of a tread pattern according to the invention with time when traveling on a wet road, the length of active ridge of the pattern in the imprint corresponding to each level of wear is preferably defined as being at least 50% of the length of active ridge on the rolling surface of the tread in the new state.
In order that the tread maintains sufficient shearing and flexural strength when subjected to the contact stresses, and this despite the presence of a very large number of cutouts, it is judicious to arrange the connecting element(s) of each of said cutouts such that the distance between the ridge closest to the rolling surface in the new state and the points of the contour of the intersection surface of said element on said wall is at most 60% of the height h of the cutout and preferably between 40% and 60%.
Another substantial improvement of the pattern of a tread according to the invention consists in effecting a plurality of cutouts provided with a large number of connecting elements distributed fairly regularly in each of the cutouts so as to ensure good regularity of the effect induced on the rigidity of the rubber elements defined by the walls of the cutout.
A tread in accordance with this latter improvement comprises a plurality of cutouts, each cutout being defined by the space defined principally between two opposing walls, said walls being perpendicular to or oblique to the rolling surface, and having a depth, measured as the distance in a radial direction of the points of the contour of said cutout which are farthest from the rolling surface of the new tire, at most equal to the thickness E of the tread. The tread is characterized in that:
the two main walls of at least one cutout located in the part of the tread affected by the loading on the ground of the tire during travel are connected by a plurality of rubber connecting elements, said connecting elements having with each of the walls a connection surface SE, said connection surface SE on each of the walls being at least 10% of the surface ST of said wall,
each connecting element having with each of the walls an intersection surface SE at most equal to Exc3x97E/20,
and in that the surface SG, defined by the geometric contour L of minimum length and a enveloping the entire connection surface SE, is at least 70% of the surface ST, of said wall.
This tread imparts particularly homogenous and regular performance to the tire, no matter what the degree of wear of said tread.
Another object of the invention is to propose a method for the production of a tread provided with a plurality of cutouts, said cutouts being provided with at least one connecting element. It clearly emerges that the conventional methods of molding cutouts are not suitable for producing such tread patterns readily and with sufficient precision, because the fact of arranging at least one connecting element beneath the rolling surface of the tread results in it being impossible to demold said tread when a conventional molding process is used which uses metal blades to mold incisions.
One object of the invention is to propose a method for obtaining a tread for a tire, comprising at least one cutout, the main walls of which are connected by at least one connecting element, eliminating the problems of demolding. The method for the production of a tread of thickness E according to the invention comprises the following steps:
a) production of inserts in an appropriate material, said inserts having the general shape of the cutouts desired in the tread, and of thickness equal to the width of said cutouts;
b) removal of material in each insert to obtain (an) orifice(s) in a distribution selected beforehand, each orifice having a shape equivalent to the shape of a connecting element;
c) insertion of the inserts produced in the preceding stages into a strip of rubber;
d) molding under pressure in a mold having the dimensions of the tread desired, during which the rubber mix of the tread, which has become more fluid, fills the orifices of the inserts so as to form connecting elements between the rubber walls adjacent to each insert;
e) vulcanization of said rubber strip followed by demolding.
Of course, this tread may be made in the form of a strip of predetermined length, or in the form of a closed ring intended to be placed radially on the outside of the blank of a tire during the manufacture thereof or on a tire being recapped.
One variant of the method which has just been described may consist in effecting steps a) to c) and placing the non-vulcanized, non-molded rubber strip on a tire blank before proceeding with the molding and vulcanization of the tire and tread assembly thus assembled.
More generally, this method, which consists in inserting inserts into a strip of rubber, may also be applied in the case of the manufacture of a tread comprising a plurality of inserts without orifice in order to mold a plurality of incisions which do not comprise connecting elements.
So as to facilitate the insertion of the inserts into the rubber strip, another variant of the method described is proposed, consisting in producing, in a non-vulcanized rubber strip, cutouts of dimensions substantially equal to the dimensions of the cutouts provided with at least one connecting element between the walls of said cutouts.
The material forming the inserts and filling the cutouts which are provided with at least one connecting element is selected so as to be able to be eliminated at least in part, that is to say, at least close to the rolling surface when new, and gradually during traveling in order permanently to maintain a great length of ridges.
It has been found that paper pulp is a filling material particularly suited to this use, because this material is characterized by very low cohesion once it is in the presence of water for a sufficient time, and therefore has the advantage of being able to be eliminated gradually during travel of the tire, or alternatively of being able to be eliminated after having been put in the presence of water and before any travel, all the more easily when the cutouts provided with at least one connecting element have a thickness of at least 0.4 mm.
One variant of this method consists in selecting as the material composing the inserts a material having a melting point sufficiently close to the vulcanization temperature of said tread to become fluid only towards the end of the duration of said vulcanization, taking into account the gradual increase in temperature of the rubber forming said tread, so as to permit the removal of said filling material, for example by suction or by blowing after vulcanization of said tread.
Preferably, the filling material is an alloy having a low melting point, having the advantage of being rigid during the shaping phase of the tread and during the vulcanization phase before becoming fluid at the end of the vulcanization of the tread so as to permit the removal of said alloy and possibly recycling it for a similar use.
The method which has just been described may also be succeeded by a grinding operation affecting virtually only the surface of the new tread and intended to make a plurality of ridges appear very distinctly on the rolling surface of said tread. Another way of obtaining a good surface state immediately on emerging from the mold may consist in arranging all the cutouts provided with at least one connecting element beneath the rolling surface of the tread and molding said tread in a mold comprising a plurality of relief elements intended to mold a plurality of cutouts on the rolling surface and the depth of which is slightly greater than the smallest of the distances between the cutouts provided with at least one connecting element of the rolling surface. In this way, the tread, as it wears down, will gradually reveal a plurality of cutouts beneath the rolling surface of said new tread.